Formation of p-n junctions



Dec. 26, 1961 P. HUNTER FORMATION OF PN JUNCTIONS Original Filed April 19, 1952 L A R T U E N R 0 M U U c A V OR REDUCING ATMOSPHERE SOURCE OF ELECTRICITY FIG.I

VACUUM OR NEUTRAL OR REDUCING ATMOSPHERE FIG. 2

SOURCE OF ELECTRICITY VACUUM 0R NEUTRAL 0R REDUCING ATMOSPHERE INVENTOR LLOYD P. HUNTER If. i

FIG.3

' (Briginal application 19, 1 .952, Ser.

a a o ddl ihl r of! M. 5 r 9 7 n.) file ff i a. seagull v A t -Cfi iarerrteol Dec. 20, 195i for the continually increasing amount of heat abstracted 7 Y 3,934,819 7 WWW n from the melt through the growing crystal.

l tjellhlr-l'hfirl n ?-N lsJ Accordingly, the principal feature of this invention is loyd P. Hunter, Ponghreepsie, N.Y., assig tor national Business Machines Corporation, New Yarn, N.Y., a corporation of New Yorir No. 2%,222, new Patent No. 2,8?L1tl5, dated Early 23, 1%9. Bivided and this application Dec. 26, 1958, er. No. 733,4llti 5 Claims. 148-45) This application is a division of US. Patent No. 2,897,105.

This invention relates to the formation of P-N junctions and more particularly to methods of forming P-N junctions in semiconductors, I

As is now well known in the alt, semiconductor diodes and transistors (including triodes, tetrodes, pentrodes, etc.) have many uses in the field of electronics and in many applications are preferred over their thermionic or vacuum tube counterparts. Initially these semiconductor devices comprised a small block of semiconductor material to one surface of which was applied one or more point contact or rectifying electrodes. Later it was discovered that two or more layers of alternately N- and P-type semiconductor material might be utilized, together with contacts of the ohmic type, one for and connected to each layer. See for example, Shockley et al., P-N Junction Transistors, Physical Review, vol. 83, pp. 161-162, July 1, 1951. As there is pointed out, in N- conductivity type semiconductor material the charges normally available for carrying current are negative, i.e., electrons, whereas in P-conductivity type semiconductor material the charges normally available for carrying current are positive i.e., holes.

Known methods of forming P-N junctions in semiconductors have certain disadvantages. One method, which may be termed the difiusion method, involves the placement of a quantity of P- or N-type impurity element in physical contact with the opposite sides of a thin wafer on N- or P-type semiconductor, respectively, of the proper resistivity value and then heating the mass to a temperature sufiicient to cause the impurity to diffuse into the interior of the wafer. This heating is terminated just before the center layer of the wafer is converted from its original conductivity to that of the impurity. One marked disadvantage of this method is the lack of independent control of the resistivity of the two regions converted by the impurity. Another is the relatively wide boundaries or junctions between the regions of different conductivity type.

Another method, which may be termed the pulling method, involves initially making contact between one end of a seed crystal and a melt of the same semiconductor material, maintaining a thermal gradient in the apparatus so that the melting point is at the contacted surface and slowly withdrawing the seed crystal so that the meniscus freezes as it rises from the melt. This method, used primarily for growing single crystals, may also be used to create P-N junctions by changing the conductivity type of the melt at intervals as the seed crystal is withdrawn. See: Teal et al., Growth of Germanium Single Crystals Containing P-N Junctions, Physical Review, vol. 81, p. 637, February 15, 1951. Disadvantages of this method include (1) the degree of mechanical stability of the melt required to insure success is very great, and the slightest vibration transmitted to the relatively large mass of the melt may cause imperfect junctions, (2) the required thermal gradient is extremely difiicult to maintain since as the process continues, the level of the melt changes and necessitates an adjustment in position of the thermal gradient, and (3) the rate of withdrawal of the crystal must be carefully controlled and adjusted to compensate the provision of new methods of forming P-N junctions in semiconductors which do not have these disadvantages, which methods are characterized by placing abutting bodies of the same semiconductor material but of opposite conductivity types in an atmosphere which is non-contaminating and non-reacting with the semiconductor material and applying heat to at least one body to raise it to the melting point of the semiconductor material.

Other features of this invention will be pointed out in the following description and claims and illustrated in the accompanying drawings, which disclose by way of example the principle of this invention and the best mode which has been contemplated of applying that principle.

In the drawings:

PEG. 1 illustrates in schematic form and partially in cross section one method of forming P-N junctions in semiconductors in accordance with this invention.

FIG. 2 illustrates in schematic form and partially in cross section an alternative method of forming P-N junctions in semiconductors in accordance with this invention.

FIG. 3 illustrates, partially in cross section, apparatus for carrying out the method illustrated somewhat more schematically in EEG. 2.

Referring now to FIG. 1, a body of semiconductor material of either 1 or P-type conductivity in the form of a small rod 1th is placed in a pure graphite crucible 12. By way of example, this rod may be of N-type germanium approximately one square millimeter in cross section and of a length such that the top of rod 10 remains about of an inch below the top of the crucible. Crucible 12 is now filled with reduced semiconductor material 14 of the opposite conductivity type, e.g., germanium metal powder containing the appropriate amount of P-conductivity type impurity. A radiant heater 16 is then placed immediately above the crucible as shown, this heater also being formed of pure graphite in order that neither it or the crucible will introduce objectionable impurities into the semiconductor material. The apparatus thus far described is surrounded by an atmosphere envelope 18, which may, for example, be of quartz, and the space within envelope 18 then made non-contaminating and non-reacting as regards the semiconductor material. This may be accomplished either by evacuation to produce a vacuum or by filling the envelope with a neutral or reducing atmosphere, e.g. purified helium or hydrogen, respectively.

Radiant heater 16 is fed from an external source of electricity 2t) and the temperature of the upper surface of the melt and the crucible 12 raised to the melting point of the germanium powder, approximately 946 C. Since heat is applied only from above as shown, there will be a steep thermal gradient in both the material and the crucible. It is, therefore, possible to maintain the temperature corresponding to the melting point of the semiconductor material throughout the powder and at the upper surface of the N-type germanium rod 19, while the remainer of rod ill is maintained below the melting point of the semiconductor material. After powder 14- has been completely melted, the temperature is slowly lowered until the crystal structure of the original germanium rod it) extends itself through the new P-type region formed from powder 14, and the whole mass then becomes a single crystal. During this cooling process, the temperature may be reduced initially at a reasonably rapid rate, e.g. 10 per minute, until a temperature of 550 C. is reached. The mass should then be maintained at this temperature for approximately sixteen hours before it is allowed to cool further.

if added junctions are desired, i.e., to form an N-P-N or a P-N-P block or body, the process above described may be repeated with powder of the desired conductivity type placed against the desired surface of opposite conductivity' type of the body and the melting and freezing 'or cooling process above described repeated. It is, of course, obvious that this process may be repeated as many times as desired to produce not only a semiconductor diode or triode body, but also bodies for tetrodes, pentodes, etc.

While the above example has been given in terms of a rod of N-type germanium material and powder of P-type germanium material, if desired the rod may be of P-conductivity type germanium and the powder of N-conductivity type germanium. Also the powder may be replaced by a solid body. Further, the method is not limited to any specific semiconductor material, although only one semiconductor material may be used at a time. For example, silicon may be utilized instead of germanium and P-N junctions formed therein in the same manner, although higher temperatures are then required in View of the higher melting point of silicon.

in FIG. 2 is illustrated schematically an alternative method of forming P-N junctions in semiconductors which expand on freezing, e.g. germanium. Similar elements in all figures are designated by the same reference numerals or by corresponding primed reference numerals. The major difference between the methods of F163. 1 and 2 is that in FIG. 2 pressure is applied to both bodies (of the same semiconductor material but of opposite conductivity types) during the formation of the P-N junction as described hereinafter. Again, the description of the method of FIG. 2 will be given in terms of N- and P-conductivity type germanium. However, other semiconductor materials which expand upon freezing may be utilized.

As shown in FIG. 2, two wafers it) and 14' of germanium of opposite conductivity types are pressed to gether between two opposed members 22. and 2.4 as indicated by the force arrows 26. Members 22 and 2dare again made of pure graphite in order to prevent any undesired impurities from contaminating or reacting with the germanium. One wafer 14 is of constant cross sectional area and is smaller than the other wafer 16' so that the surface area of water 1d abutting the opposed surface of the larger wafer lid is smaller than that surface area of the latter.

The ratio of the areas will depend upon the accuracy of the ambient temperature control. For example, if the ambient temperature can only be controlled to i2 (3., it is essential to have at least C. difference in the melting points of the two wafers for a constant applied force. This differential in melting points may be obtained either by varying the applied force, the ratio of the crosssectional areas, the absolute areas, or any combination of these since the lowering of the melting point is directly proportional to the applied pressure.

Referring again to FIG. 2, as before a pure graphite radiant heater 16 is provided to heat the semiconductor material, and the apparatus thus far described is enclosed in atmosphere envelope 18, which again is either evacuated or filled with a neutral or reducing atmosphere. An external source of electricity 20 is again connected to electric radiant heater in, which is illustrated schematically as comprising two elements but may conveniently be in the form of a circular coil surrounding the wafers and 14-.

Since the lower surface area of wafer 14' is considerably smaller than the abutting upper surface area of wafer iii as described above, it is possible, when pressure is applied to press wafers It) and 14 together to melt wafer 14' at a temperature below that at which wafer It? will melt. This is accomplished by applying a constant force to the members 22' and 2 as indicated by the force arrows 26 and slowly raising the ambient temperature of the apparatus toward the normal melting point of the semiconductor material being processed. When the melting point of wafer 14 is reached corresponding to the pressure thereon, wafer 14' and the immediately adjacent surface of wafer 10 fuse, thus increasing the abutting surface areas of wafers Ill and 14 under the action of the steadily applied force. If the temperature is raised no further, the system comes to equilibrium and the materials freeze as the cross sectional area of the abutting stuface of wafers 1th and 14' increases until the constant applied force is no longer sufficient to maintain the material molten for the temperature maintained by radiant heater 16, which temperature is less than the normal melting point for the semiconductor material.

The depth of penetration of the melted region in wafer in is limited under these conditions since the pressure drops off rapidly going from wafer 14' across the interface into Wafer iii. Since wafer 14' is of uniform cross sectional area and represents the smallest cross sectional area in the system, a uniform pressure will exist throughout wafer 14' and it will therefore melt throughout at the same temperature.

Two or more P-N junctions may be formed simultaneously in accordance with this second method by providing pairs of large wafers 2th of one conductivity type and sandwiching a small wafer 14' of opposite conductivity type between each pair of wafers it), thus two wafers and one wafer 14 would produce a P-N-P or N-P-N body, depending upon whether wafer 14 was of N- or P-type material respectively. Also a large water 1d may be sandwiched between two smaller wafers 14' of equal cross section to again produce the highest and equal pressure at each abutment between wafer 19' and a wafer 14. Alternatively, a single P-N junction may be formed as described above and the process then repeated as many times as desired by pressing, each time, an additional wafer of the proper conductivity type against the desired wafer of the opposite conductivity type, this additional wafer each time being of smaller cross sectional area than the abutting'surface of'the wafer against which it is pressed. For example, if a third N- type germanium wafer 15 were pressed against P-type germanium wafer 14, its cross sectional area should be smaller than the abutting surface of the latter, and an N-P-N body or block would result.

It is believed that if this alternative process for forming two or more P I junctions is carried out, together with a very slow freezing following a forming of each P-N junction, that the P-N boundaries are produced in a single crystal structure, whereas if a plurality of P-N junctions are produced simultaneously as first described by sandwiching layers of three or more wafers, crystal boundaries coincident with the P-N junctions are produced.

in FIG. 3, which is not to scale for the sake of clarity, is shown in somewhat more detail apparatus for carrying out the method of FIG. 2. Pressure member 22 is shown in the form of a graphite base plate sealed to atmosphere envelope is, which may for example be of quartz. Larger wafer 1%, which again may for example be of N-conductivity type germanium, is placed atop base plate 22 and smaller wafer 14, which would then be of P-conductivity type germanium, is sandwiched between wafer Jill and the second graphite pressure member 24. A quartz pressure rod 28 extends through atmosphere envelope 18 and at its lower end abuts pressure member 24. Atop its other end may be placed a suitable weight or weights 3% to produce the desired pressure at the abutting opposed surfaces of wafers lit? and 14'. In order to produce this pressure there must, of course, be a slidable seal between atmosphere envelope 18 and either base member 22 or rod 28, or else atmosphere envelope 1% must be able to be flexed sufficiently to allow the desired pressure to be exerted between pressure members 22 and 24. A pot furnace 32 partially surrounds the structure thus far described and is heated by heating elements 16', which no longer need be of graphite since they are now positioned outside atmosphere envelope 18. The conventional source of electricity for heating element 16 is not shown in this figure. As indicated, pot furnace 32 extends far enough above the plane of the abutting opposed surfaces of wafers 10 and 14' so that the desired carefully-controlled constant temperature may be maintained thereat. An apertured cover 34 for pot furnace 32 may be provided as shown to assist in maintaining this desired temperature.

By way of example, atmosphere envelope 18 may be evacuated to provide a vacuum, and wafer 10' may be 0.002 square inch in cross section and wafer 14' is approximately 0.001 square inch in cross section. A weight 30 of six lbs. then produces approximately 6,000 p.s.i. pressure at the abutting opposed surfaces of wafers 10 and 14'. At this pressure the normal melting point of germanium is reduced approximately i.e. from 946 C. to 941 C. Thus the difierence in melting point of wafers and 14' is 2.5 inasmuch as their cross sectional areas are in the ratio 2: 1. If then the temperature of wafers 10' and 14 is raised approximately 941, wafer 14' will melt and then solidify as both its surface area and cross sectional area increase. The wafers 10 and 14' may then be cooled fairly rapidly, e.g., 10 C. per minute, to 550 C. and then maintained at that temperature for approximately sixteen hours before they are further cooled.

A variation of this second method of FIGS. 2 and 3 for the formation of P-N junctions in semiconductors which expand on freezing is obtained by utilizing two wafers 10 and 14 of the same cross section but of which at least one is of reduced cross sectional area at their abutment. The greatest reduction in melting point of the semiconductor material thus again occurs only in the material immediately adjacent this abutment.

In accordance with this invention, methods have been disclosed for forming P-N junctions in semiconductors which allow the independent control of the resistivity value of the various P- and N-regions of the final crystal, which do not require great mechanical stability, and which do not require extremely involved temperature control as in the pulling method.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to a preferred embodiment it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated and in its operation may be made by those skilled in the art, without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

What is claimed is:

1. 'A method of forming P-N junctions in semiconductors comprising the steps of placing a powder of one conductivity type semiconductor material abutting a body of the same semiconductor material of the other conductivity type in a vacuum and heating the powder to the melting point of the semiconductor material.

2. A method of forming P-N junctions in semiconductors comprising the steps of placing a powder of one conductivity type semiconductor material abutting a body of the same semiconductor material of the other conductivity type in a non-contaminating atmosphere non-reacting with the semiconductor material and heating the powder to the melting point of the semiconductor material.

3. A method of forming P-N germanium junctions comprising the steps of placing a powder of P-conductivity type germanium abutting a body of N-conductivity type germanium in a vacuum, heating the powder to approximately 946 C. to melt the powder while maintaining the body at a temperature less than 946 C., thereupon reducing the temperature of the melted powder and the body until a temperature of approximately 550 C. is reached, and maintaining that lower temperature for a period of approximately sixteen hours before further cooling the melted powder and body.

4. A method of forming P-N germanium junctions comprising the steps of placing a powder of N-conductivity type germanium abutting a body of P-conductivity type germanium in a vacuum, heating the powder to approximately 946 C. to melt the powder while maintaining the body at a temperature less than 946 C. thereupon reducing the temperature of the melted powder and the body until a temperature of approximately 550 C. is reached, and maintaining that lower temperature for a period of approximately sixteen hours before further cooling the melted powder and body.

5. A method of forming P-N junctions in semiconductors comprising the Steps of placing a solid body of a semiconductor material of a first conductivity type in a non-contaminating container, placing a powdered semiconductor material of a second conductivity type above the solid body in the container, and heating the powder to the melting point of the solid body in a non-contaminating atmosphere.

References Cited in the file of this patent UNITED STATES PATENTS 2,631,356 Sparks etal Mar. 17, 1953 2,644,852 Dunlap July 7, 1953 2,701,326 Pfann et al. Feb. 1, 1955 2,708,255 White May 10, 1955 2,708,646 North May 17, 1955 2,859,140 Clarke Nov. 4, 1958 

1. A METHOD OF FORMING P-N JUNCTIONS IN SEMICONDUCTORS COMPRISING THE STEPS OF PLACING A POWDER OF ONE CONDUCTIVITY TYPE SEMICONDUCTOR MATERIAL ABUTTING A BODY OF THE SAME SEMICONDUCTOR MATERIAL OF THE OTHER CONDUCTIVITY TYPE IN A VACUUM AND HEATING THE POWDER TO THE MELTING POINT OF THE SEMICONDUCTOR MATERIAL. 